Probing Molecular-Level Dynamic Interactions of Dissolved Organic Matter with Iron Oxyhydroxide via a Coupled Microfluidic Reactor and an Online High-Resolution Mass Spectrometry System

The interactions between dissolved organic matter (DOM) and iron (Fe) oxyhydroxide are crucial in regulating the biogeochemical cycling of nutrients and elements, including the preservation of carbon in soils. The mechanisms of DOM molecular assembly on mineral surfaces have been extensively studied at the mesoscale with equilibrium experiments, yet the molecular-level evolution of the DOM-mineral interface under dynamic interaction conditions is not fully understood. Here, we designed a microfluidic reactor coupled with an online solid phase extraction (SPE)-LC-QTOF MS system to continually monitor the changes in DOM composition during flowing contact with Fe oxyhydroxide at circumneutral pH, which simulates soil minerals interacting with constant DOM input. Time-series UV-visible absorption spectra and mass spectrometry data showed that after aromatic DOM moieties were first preferentially sequestered by the pristine Fe oxyhydroxide surface, the adsorption of nonaromatic DOM molecules with greater hydrophobicity, lower acidity, and lower molecular weights (<400) from new DOM solutions was favored. This is accompanied by a transition from mineral surface chemistry-dominated adsorption to organic-organic interaction dominated adsorption. These findings provide direct molecular-level evidence to the zonal model of DOM assembly on mineral surfaces by taking the dynamics of interfacial interactions into consideration. This study also shows that coupled microfluidics and online high-resolution mass spectrometry (HRMS) system is a promising experimental platform for probing microscale environmental carbon dynamics by integrating in situ reactions, sample pretreatment, and automatic analysis.

Automated summary

The interactions between dissolved organic matter (DOM) and iron (Fe) oxyhydroxide play a crucial role in the biogeochemical cycling of nutrients and carbon preservation in soils. This study used a microfluidic reactor coupled with online mass spectrometry to investigate the molecular-level evolution of DOM-mineral interface under dynamic interaction conditions. The results showed that the preferential adsorption of aromatic DOM molecules by Fe oxyhydroxide surface transitioned to the adsorption of nonaromatic DOM molecules with greater hydrophobicity, lower acidity, and lower molecular weights from new DOM solutions. This study provides molecular-level evidence supporting the zonal model of DOM assembly on mineral surfaces and demonstrates the potential of microfluidics and high-resolution mass spectrometry for studying environmental carbon dynamics.